1. Field of the Invention
The present invention relates to an inner-focusing-type photographing lens system which (i) is suitable for a photographing camera or a video camera, (ii) performs focusing by moving an inner portion of the lens system, i.e., lens groups other than the most object-side lens group, (iii) has a bright F-number of about 2.5, and (iv) the full angle-of-view 2xcfx89 is about 21 degrees.
2. Description of the Related Art
In a fast intermediate-telephoto lens system which has (i) an F-number of less than 2.8, and (ii) the full angle-of-view of about 18 to 28 degrees, the entire length of the lens system becomes longer since the focal length thereof is relatively long. Moreover, due to the bright F-number of the above intermediate-telephoto lens system, the lens diameters thereof become larger, so that the lens system tends to be larger and heavier. Consequently, for example, in an auto-focus (AF) camera, since a heavy load is exerted on the lens-driving system for moving lens groups in the lens system, the speed of focusing becomes slower. Because of these reasons, an inner-focusing-type photographing lens system, in which only an inner portion of the lens system is moved in order to perform focusing has been proposed, as disclosed in, e.g., Japanese Unexamined Patent Publication (JUPP) No. Hei-4-255813, and JUPP No. Hei-5-157964.
In the above inner-focusing-type photographing lens system, in order to reduce the burden of the correcting of aberrations in a focusing lens group, and to reduce the fluctuation of aberrations therein upon focusing, it is preferable to reduce aberrations in a lens group in front of the focusing lens group. Accordingly, the lens group in front of the focusing lens group is commonly constituted by three positive lens elements and one negative lens element. On the other hand, in such a fast intermediate telephoto lens system, the diameter of the frontmost lens group tends to be larger due to a relatively wider angle-of-view and a smaller F-number. Still further, there are cases where a filter is attached to the front of the photographing lens system. In those cases, if the diameter of the frontmost lens group is larger, the weight and cost of accessories, such as a filter and the like, have to be increased as well. Moreover, an increase in the diameter of the frontmost lens group causes an increase in the weight of the entire lens system. Consequently, portability of a photographing lens (barrel), to which the above inner-focusing-type photographing lens system is provided, becomes worse. Therefore the frontmost lens group with a smaller diameter is required.
An object of the present invention is to provide a photographing lens system which (i) has a relatively bright F-number of about 2.5, (ii) the full angle-of-view 2xcfx89 is about 21 degrees, and (iii) can reduce the diameter of the frontmost lens group. Another object of the present invention is to provide an inner-focusing-type photographing lens system which can reduce the number of lens elements of a lens group in front of the focusing lens group.
In order to achieve the above-mentioned objects, there is provided a photographing lens system including a positive first lens group, a negative second lens group, and a positive third lens group, in this order from the object. The photographing lens system is arranged to move the negative second lens group to the image, upon focusing from infinity to an object at a shorter distance. The positive first lens group includes a biconvex positive lens element, a biconvex positive lens element, and a biconcave negative lens element, in this order from the object. The second lens group includes cemented lens elements constituted by a positive meniscus lens element having the concave surface facing toward the object and a negative biconcave lens element, in this order from the object. Furthermore, the photographing lens system satisfies the following conditions:
0.020 less than dX/f less than 0.200xe2x80x83xe2x80x83(1)
(n1+n2)/2 greater than 1.52xe2x80x83xe2x80x83(2)
(xcexd1+xcexd2)/2 greater than 55xe2x80x83xe2x80x83(3)
wherein
dX designates the traveling distance of the second lens group, the distance of which corresponds to an object in an in-focus state at infinity to an object in an in-focus state at the shortest photographing distance;
f designates the focal length of the entire photographing lens system when an object at infinity is in an in-focus state;
n1 designates the refractive index of the d-line of the most object-side positive lens element in the first lens group;
n2 designates the refractive index of the d-line of the most object-side positive lens element in the second lens group;
xcexd1 designates the Abbe number of the most object-side positive lens element in the first lens group; and
xcexd2 designates the Abbe number of the most object-side positive lens element in the second lens group.
The photographing lens system preferably satisfies the following condition:
0.58 less than f1/f less than 0.72xe2x80x83xe2x80x83(4)
wherein
f1 designates the focal length of the first lens group; and
f designates the focal length of the entire photographing lens system when an object at infinity is in an in-focus state.
Furthermore, the photographing lens system preferably satisfies the following condition:
0.12 less than f/f1-2 less than 0.29xe2x80x83xe2x80x83(5)
wherein
f designates the focal length of the entire photographing lens system when an object at infinity is in an in-focus state; and
f1-2 designates the combined focal length of the first lens group and the second lens group when an object at infinity is in an in-focus state.
According to another aspect of the present invention, there is provided a photographing lens system including a positive first lens group, a negative second lens group, and a positive third lens group, in this order from the object. The photographing lens system is arranged to move the negative second lens group to the image, upon focusing from infinity to an object at a shorter distance. The positive first lens group includes a biconvex positive lens element, a biconvex positive lens element, and a biconcave negative lens element, in this order from the object. The second lens group includes cemented lens elements constituted by a positive meniscus lens element having the concave surface facing toward the object and a negative biconcave lens element, in this order from the object. The third lens group includes cemented lens elements constituted by a negative lens element having a concave surface facing toward the object and a positive lens element, and a positive lens element, in this order from the object.
Furthermore, the photographing lens system satisfies the following conditions:
0.020 less than dX/f less than 0.200xe2x80x83xe2x80x83(1)
(n1+n2)/2 greater than 1.52xe2x80x83xe2x80x83(2)
(xcexd1+xcexd2)/2 greater than 55xe2x80x83xe2x80x83(3)
wherein
dX designates the traveling distance of the second lens group, the distance of which corresponds to an object in an in-focus state at infinity to an object in an in-focus state at the shortest photographing distance;
f designates the focal length of the entire photographing lens system when an object at infinity is in an in-focus state;
n1 designates the refractive index of the d-line of the most object-side positive lens element in the first lens group;
n2 designates the refractive index of the d-line of the most object-side positive lens element in the second lens group;
xcexd1 designates the Abbe number of the most object-side positive lens element in the first lens group; and
xcexd2 designates the Abbe number of the most object-side positive lens element in the second lens group.
The photographing lens system preferably satisfies the following condition:
0.58 less than f1/f less than 0.72xe2x80x83xe2x80x83(4)
wherein
f1 designates the focal length of the first lens group; and
f designates the focal length of the entire photographing lens system when an object at infinity is in an in-focus state.
Furthermore, the photographing lens system preferably satisfies the following condition:
0.12 less than f/f1-2 less than 0.29xe2x80x83xe2x80x83(5)
wherein
f designates the focal length of the entire photographing lens system when an object at infinity is in an in-focus state; and
f1-2 designates the combined focal length of the first lens group and the second lens group when an object at infinity is in an in-focus state.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-342857 (filed on Nov. 10, 2000) which is expressly incorporated herein in its entirety.